There are image forming apparatuses in which toner images carried on an image carrier are transferred onto recording sheets, while transfer areas of the image forming apparatuses nip the recording sheet between the image carrier and transfer sections (being transfer rollers).
Further, on said image forming apparatuses, when the recording sheet enters the transfer area or separates from the transfer area, on which area the image carrier and the transfer section are structured to be in contact with each other, adverse impulsive vibration occurs. Said impulsive vibration causes instantaneous speed-fluctuations of the image carriers, whereby while the images are formed, the images receive an adverse affect, which is a well known matter.
The thicker the recording sheet, the more drastically the velocity fluctuation occurs, when the recording sheet enters or separates from the transfer area. By said velocity fluctuation, notable adverse deterioration of image quality may locally occur, such as image transferring slippage on the transfer section, or uneven exposure of the image.
Patent documents, listed below, disclose the countermeasures against the instantaneous speed-fluctuation of the image carriers, which occur due to the vibrations which occur around the transfer area, when the recording sheet enters the transfer area or draws away from the transfer area.    Patent Document 1: Unexamined Japanese Patent application publication 2001-265,127, and    Patent Document 2: Unexamined Japanese Patent application publication 2004-61,882
According to Patent Document 1, a structure to mechanically control an impulse is disclosed, so that any adverse effect of the impulse is controlled. However, since a specific mechanical structure is necessary, an apparatus increases in cost.
According to Patent Document 2, an intermediate transfer belt and a secondary transfer belt are individually rotated by a separate motor, while these belts are driven at a predetermined relative speed. When a recording sheet enters a secondary transfer section, a synchronization control is temporarily stopped, so that the relative speed between the intermediate transfer belt and the secondary transfer belt is controlled not to exceed a set limit. In this case, an operation mode is assumed to be necessary, so that an additional motor will be used for the synchronization control, which does not exhibit the general versatility.
Further, in general terms, in order to control the above-described vibrations, if the structural stiffness of the apparatus is increased, the vibration can be theoretically controlled. However, from the view point of the size of apparatus, the installation position of a flywheel, the cost of the flywheel, and the total cost of the apparatus, it is difficult to increase the structural stiffness to be greater than that of the present apparatuses.
Still further, in general terms, in order to control the above-described vibrations, soft rollers, which are configured to absorb the impulses when the recording sheet enters or separates from the transfer section, may be used for the transfer rollers and feeding rollers of conveyance sections. However, said soft rollers cannot be used as the transfer rollers, from the view point of transfer efficiency and image quality.